Summary Significant evidence indicates that sphingomyelin (SM) content in the aortic wall and in the plasma is closely related to atherogenesis. High SM is an independent risk factor for human coronary heart disease and is associated with human atherosclerotic plaque inflammation. We found that inhibiting serine palmitoyl- transferase, the first enzyme for SM biosynthesis reduced plasma SM and atherogenesis in mouse models. However, mechanisms are unknown, prompting further studies exploring relationships between blocking SM synthase (SMS) and atherogenesis. Two isoforms of SMS (SMS1 and SMS2) reside downstream of serine palmitoyl-transferase and catalyze the conversion of ceramide to SM. SMS1 and SMS2 activities are co- expressed in all tested tissues, including the liver, intestine, and macrophage. Thus, neither SMS1 gene knockout (KO) nor SMS2 KO approach is sufficient to evaluate the effect of SM reduction on atherosclerosis. We will use SMS1/SMS2 double KO approach in this study. Our objective is to test our hypotheses that inhibition of total SMS activity can: a) block SM bioavailability during the process of apoB-containing atherogenic lipoprotein (BLp, i.e. VLDL and chylomicron) production; b) reduce lipid absorption and attenuate inflammation by reducing SM in cell (enterocyte and macrophage) plasma membrane lipid rafts; and c) reduce atherosclerosis progression and regression without causing steatosis (SMS inhibition-mediated ceramide accumulation could suppress lipogenesis). In this study, we will use inducible global, liver-specific, and intestine-specific total SMS KO mouse models, as well as our specific SMS inhibitors. Specific aims: 1. Evaluate effects of blocking SMS on VLDL production and catabolism. 2. Investigate the effects of blocking SMS on lipid absorption and chylomicron secretion. 3. Examine the roles of absence or inhibition of SMS in atherosclerosis progression and regression. Insights gained from the proposed studies will allow us to evaluate SMS as a target for preventing and treating atherosclerosis.